This invention relates to plasma processor which is a semiconductor fabrication apparatus, and more particularly to a plasma processor which generates plasma by the use of electron cyclotron resonance and can effect a uniform plasma processor over large area of a substrate.
FIG. 1 is a sectional view, partly in blocks, showing the construction of an example of a prior-art plasma processor disclosed in Japanese Pat. application Laid-open No. 79621/1982. The prior-art plasma processor comprises a plasma generation portion 1. This plasma generation portion 1 has a plasma generating vessel, for example, glass tube 2, a ratio-frequency waveguide 3 which accommodates the plasma generating glass tube 2 and which generates a nonuniform r.f. electric field perpendicular to an axial direction (assumed to be the z-direction), and a solenoid coil 5 which is arranged around the r.f. waveguide 3 and which is connected to a D.C. power source 4 so as to generate a nonuniform magnetostatic field in the axial direction. Radio-frequency power is fed to the r.f. waveguide 3 through a magnetron 7 which is mounted on the upper part of the r.f. waveguide 3 and which is connected to a driving power source 6. In addition, a gas is fed to the plasma generating glass tube 2 through a gas supply pipe 8.
The prior-art plasma processor further comprises a plasma reaction portion 9. In this plasma reaction portion 9, there is disposed a stage 10 on which a substrate 11 to be processed with a plasma is placed. An exhaust pipe 12 for exhausting the used gas is connected to the lower part of the plasma reaction portion 9.
The prior-art plasma processor is constructed as stated above, and it forms the plasma on the basis of electron cyclotron resonance. Therefore, this electron cyclotron resonance will be explained below:
Here, B(z) will denote the intensity of the nonuniform magnetostatic field in the axial direction. The r.f. electric power fed into the r.f. waveguide 3 by the magnetron 7 established a nonuniform r.f. electric field E.sub.rf (z) within the plasma generation portion 1 which is so shaped as to resonate in accordance with the frequency of the r.f. power. The magnetostatic field in the z-direction, which causes the electron cyclotron resonance with the r.f. electric field E.sub.rf (z) within the plasma generation portion 1, lies in a range within the plasma generation portion 1 as shown in FIG. 2. That is, a curve from point A to point B is obtained by connecting the points of magnetic field intensities at which the magnetostatic field intensity B.sub.z (z) in the z-direction cause the resonance with the r.f. electric field E.sub.rf (z).
An electron performs a well-known cyclotron motion in the magnetostatic field B, and the angular frequency .omega..sub.c of the cyclotron motion is expressed by .omega..sub.c =eB/m (where e denotes the absolute value of electronic charge, and m denotes the mass of the electron). Letting .omega. denote the angular frequency of the r.f. electric field E.sub.rf (z) in the plasma generation portion 1, when the cyclotron resonance condition of .omega.=.omega..sub.c holds, the energy of the r.f. power is continuously supplied to the electron, and the energy of the electron increases.
Under such cyclotron resonance conditions, a gas of proper gaseous pressure is introduced into the gas supply pipe 8. Then, the electrons generated in a preliminary discharge state are continuously supplied with energy from the r.f. power, to fall into a high energy state, and the plasma is developed through the process of collisions. The r.f. power is further poured into the plasma thus developed, under the resonance conditions.
Accordingly, assuming by way of example, that the gas introduced through the gas supply pipe 8 is SiH.sub.4, the r.f. power is properly adjusted in addition to the pressure of the gas, whereby the types, concentrations and/or energy levels of respective ions such as Si.sup.+, SiH.sub.2.sup.+ and SiH.sub.3.sup.+ can be controlled, and simultaneously, the types, concentrations or/and energy levels of radicals such as Si.sup.* and SiH.sub.x.sup.* can be controlled.
Meanwhile, an axial force F.sub.z given by the following equation acts on the electron in the presence of both the nonuniform magnetostatic field B(z) and the nonuniform electric Field E.sub.rf (z), so that the electron is accelerated in the axial direction. ##EQU1## where .mu. denotes a magnetic moment, B a magnetic flux density, z a distance in the axial direction, .omega..sub.0 the energy of the circular motion of the electron, B.sub.0 a magnetic flux density in the plasma generation portion 1, m the mass of the electron, and M the mass of the ion.
Accordingly, the electrons in the plasma generated by the plasma generation portion 1 in FIG. 1 are axially accelerated toward the plasma reaction portion 9. In consequence, an electrostatic field E.sub.0 (z) which accelerates the ions is established in the axial direction within the plasma. This electrostatic field E.sub.0 (z) accelerates the plasma as a whole in the axial direction, so that a plasma current 13 extending in the axial direction appears in the plasma reaction portion 9. Since magnetic lines of forces created by the solenoid coil 5 come to have components of an r-direction in the plasma reaction portion 9, the plasma current 13 spreads along the magnetic lines of forces.
Such a plasma processor can be applied to various surface processes including plasma etching, plasma CVD and plasma oxidation, and can effectively perform these processes.
With the prior-art plasma processor utilizing electron cyclotron resonance, the z-directional component B.sub.z (z) of the magnetostatic field causing the resonance with the r.f. electric field E.sub.rf (z) does not cover the entire region of the plasma generation portion in the radial direction (r-direction) thereof as seen from FIG. 2. This leads to the problem that, in general, uniformity in the plasma process is difficult to attain. By way of example, when a film is formed by the plasma CVD, the distribution of the thickness of the film becomes nonuniform as illustrated in FIG. 3.